1. Technical Field
This invention relates fuel cells, and particularly to the operation of fuel cells.
2. Background Information
It is generally agreed that one difficulty with utilizing fuel cell systems that operate out of doors, such as to power a vehicle, is starting and operating the fuel cell at temperatures below the freezing point of water. Freezing provides potential mechanical damage as a consequence of the expansion of ice, and presents problems due to the inseparability of water and the fuel cell processes. For example, fuel cells are typically cooled by circulating water therethrough, and during operation, water is required for humidification of the reactant gases. A problem particularly related to the use of fuel cells in vehicles is the need to be able to quickly start the vehicle after exposure to freezing temperatures that have caused water within the fuel cell system to freeze. Heretofore, various methods for starting “frozen” fuel cells have concentrated on providing heat for periods of time to appropriate portions of the system, either by reaction or by combustion of fuel, or by means of battery power. Other efforts are directed toward processes designed to accelerate the rate at which a fuel cell stack will heat up to above-freezing temperatures, as a consequence of its own operation. In U.S. Pat. No. 5,798,186, the fuel cell is warmed up simply by connecting a load across it while stochiometric fuel, oxidant are supplied to the stack. In U.S. Pat. No. 6,329,089, individual fuel cells at −5° C. started with room temperature hydrogen and air reached 0.5 amps per cm2 in five minutes. This is unsatisfactory for vehicles, such as automobiles, which must be operating in less than one minute after initiating startup, at temperatures as low as −40° C.
With proton exchange membrane (PEM) fuel cells, such as those described in U.S. Pat. No. 6,024,848 to Dufner and incorporated herein by reference, there is the additional problem that areas of the PEM or of a water transport plate (WTP) within the cell may dry out during the period of start-up and warm-up due to the lack of water available for humidification of the reactants. Such dry out may result in the mixing of reactants or the degradation of materials.